A major problem in the regulation of metabolism is to understand those factors relevant to the determination of intracellular enzyme concentrations in eukaryotic cells. In contrast to protein synthesis, relatively little is known about controls of protein turnover. One hypothesis assigns a major role to apoprotein - cofactor interaction in the modulation of intracellular turnover. The role of cofactor binding will be assessed by studying turnover of: representative PLP enzymes of the sulfur amino acid pathway: and mitochondrial carboxylases in cells expressing defects in biotin transport or metabolism. Studies are designed to define parameters relevant to enzyme turnover in these representative systems. Experimental goals will include characterization of the inactivation system itself, and of early products of the degradation reactions. The results will have relevance to the understanding of the control of intracellular enzyme levels in eukaryotic cells, as well as to a clarification of mechanisms obtaining in several inborn errors of metabolism.